Integrated absolute and differential pressure transducer

ABSTRACT

A method and apparatus integrates differential pressure measurements and absolute pressure measurements to provide a continuous absolute pressure profile over a wide range of pressures on a single integrated scale. The absolute pressure measurements and differential pressure measurements are obtained, and a correlation factor between the absolute pressure measurements and the differential pressure measurements is determined. The correlation factor is used to normalize the differential pressure measurements to virtual absolute pressure values on a common absolute pressure scale with the absolute pressure measurements. An absolute pressure profile over a wide pressure range includes the absolute pressure measurements in a portion of the range where the absolute pressure measurements are accurate, and it includes the virtual absolute pressure values in another portion of the range where the differential pressure measurements are accurate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/158,917, entitled “Integrated Absolute And Differential PressureTransducer, filed Jun. 21, 2005, which is a continuation of U.S. patentapplication Ser. No. 10/721,817, filed Nov. 24, 2003, now issued as U.S.Pat. No. 6,909,975.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to pressure sensors and, more particularly, toan integrated absolute and differential pressure sensor that can providenormalized, absolute and differential pressure measurements and outputsover a broad range of sub-atmospheric, atmospheric, andsuper-atmospheric pressures.

2. State of the Prior Art

In some process, control, or monitoring applications, it would be verybeneficial to have the capability of sensing pressure and providingaccurate and repeatable pressure measurement or control outputs over abroad pressure range, such as from 10⁻⁸ torr or lower to 10³ (1,000)torr or higher. For example, in a physical vapor deposition (PVD) orchemical vapor deposition (CVD) vacuum process chamber for depositingthin films of semiconductor materials on substrates or wafers tofabricate semi-conductor devices, a common deposition process practicemay be some variation of the following: (i) Load the substrate or waferinto the vacuum process chamber at atmospheric pressure (e.g., about600-770 torr); (ii) Close and seal the process chamber and evacuate itto 10⁻⁷ torr or less and hold it there for some period of time to removeall of the air, water vapor, and other potential contaminants, (iii)Back-fill the chamber with inert or over-pressure gas to bring theprocess chamber back up to about 10⁻³ torr, where it is maintained whileprocess and carrier gasses are fed into the chamber to react orotherwise form a thin film of the desired semiconductor material(s) onthe substrate or wafer, while effluents comprising gaseous by-products,unreacted and excess process gasses, and carrier gasses are drawn out ofthe process chamber; (iv) Stopping the process gasses; and (v)Back-filling the process chamber to increase the pressure in the chamberback to atmospheric pressure so that the chamber can be opened to removethe processed device.

Another approach is to keep the process chamber at the very low processpressure (vacuum) range used for the deposition processes, while aseparate, often smaller, load lock chamber is used to handle the wafersbefore and after processing, i.e., to cycle between atmospheric pressureand process pressure to move the wafers into and out of the processchamber. The process chamber, when used with such a load lock, is onlyexposed to atmospheric pressure, therefore, when it is opened forservicing.

Such vacuum process and load lock systems currently require a pluralityof different kinds of individual pressure sensors to measure and/orcontrol pressures over such large ranges. For example, hot cathodepressure sensors are considered to be accurate and dependable forabsolute pressure measurements in a range of about 5×10⁻¹⁰ to 5×10⁻² ,but they are not useful for pressures above 5×10⁻² torr and have to beturned off to avoid burning out the filaments inside the hot cathodegauges. On the other hand, conventional convection pirani pressuresensors have absolute pressure measuring capabilities in a range ofabout 10³¹ ³ torr to 1,000 torr, but they are not useful for pressuresbelow 10³¹ ³ torr, and they have a flat zone in a range of about 10 to1,000 torr in which accuracy is low. A micropirani pressure sensor, suchas the micropirani pressure sensor described in published U.S. patentapplication Ser. No. 09/907,541, now U.S. Pat. No. 6,672,171, which isincorporated herein by reference, can extend that range down to about10³¹ ⁵ torr and alleviate the flat zone, but that range still is notsufficient alone for many processes.

Further, absolute pressure sensors are problematic in applications suchas the vacuum process chamber described above, because, while it may bedesirable to open the process chamber door at or very near ambientatmospheric pressure, ambient atmospheric pressure varies, depending onelevation above sea level, weather patterns, and the like, so anyparticular set point of an absolute pressure sensor is unlikely to matchatmospheric pressure consistently. Thus, a differential pressure sensormay be required in addition to the one or two different kinds ofabsolute pressure sensors described above to provide the requiredprocess pressure measurements and controls, which still does not addressthe flat zone problems, especially where critical process operations arerequired or desired at pressures that coincide with such flat zones.

The combination absolute and differential pressure transducer describedin the U.S. patent application Ser. No. 09/907,541, published on Jan.16, 2003, (now U.S. Pat. No. 6,672,171, issued on Jan. 6, 2004) providesa beneficial combination of an absolute pressure sensor with adifferential pressure sensor for controlling the opening or closing ofinterior and exterior doors and other functions of load locks for vacuumprocessing chambers of transfer chambers. However, the absolute pressuremeasurements and the differential pressure measurements are separatefrom each other, and it provides no way to obtain or track absolutepressures above the absolute pressure measuring capability of theabsolute pressure sensor and through the differential pressure sensorranges. Of course, one or more different types of absolute pressuresensors could be added to the combination to provide higher absolutepressure measurements in the higher, differential pressure measurementranges, but such additional pressure transducers add to the cost of theprocess equipment and are still not truly integrated in their respectivemeasurements. Many process chamber operators and quality controltechnicians would like to see an entire process pressure profile on asingle absolute pressure scale from atmospheric pressure or higher anddown to the lowest vacuum pressure and then back up through those rangesto atmosphere again.

SUMMARY OF THE INVENTION

A general object of this invention, therefore, is to provide a methodand apparatus for measuring and/or controlling pressures over widepressure ranges with absolute and/or differential pressure outputsextending in an integrated manner over such ranges.

A more specific object of the invention is to provide real time absolutepressure measurements on a single scale extending from above atmosphericpressure to very low vacuum pressures.

A more specific object of this invention is to provide integratedabsolute and/or differential pressure measurement and outputcapabilities over a pressure range of 10⁻⁸ torr or lower to 10³ torr orhigher with as few as one or two absolute pressure sensors and onedifferential pressure sensor.

To achieve the foregoing and other objects, a method and apparatus formeasuring absolute pressure in a chamber includes determining acorrelation factor between absolute and differential pressuremeasurements taken simultaneously at a pressure where the absolutepressure in the chamber can be measured accurately and reliably, andthen adjusting differential pressure measurements with the correlationfactor to provide virtual absolute pressure measurements. The absolutepressure measurements are used in chamber pressure ranges where they aremore accurate and reliable than the virtual absolute pressuremeasurements, and the virtual absolute pressure measurements are used inchamber pressure ranges where they are more accurate and reliable thanthe absolute pressure measurements. The correlation factor isre-determined periodically to adjust for changes in atmosphericpressure. The scope of the invention is defined in the claims below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the preferred embodiments of the presentinvention, and together with the written description and claims, serveto explain the principles of the invention. In the drawings:

FIG. 1 is a diagrammatic view of a load lock for a vacuum processchamber fitted with an absolute pressure sensor and a differentialpressure sensor for measuring absolute pressures in a chamber on acontinuous scale over a large pressure range according to thisinvention;

FIG. 2 is a chart showing absolute pressures juxtaposed to example sealevel and high plains atmospheric pressures to illustrate a problemaddressed by, and some fundamentals of, this invention;

FIG. 3 is a logic flow chart illustrating an algorithm used in thisinvention to extend absolute pressure measurement range into thedifferential pressure sensor range;

FIG. 4 is a bifurcated bar chart illustrating the effective differentialand absolute pressure ranges of two pressure sensors, one differentialand the other absolute, for producing absolute pressure measurementsover a wide range of pressures on a single absolute pressure scaleaccording to this invention;

FIG. 5 is a pressure profile of a conventional load lock control cycleillustrating an example application of the integrated combinationabsolute and differential pressure transducer illustrated in FIGS. 1-4;

FIG. 6 is a logic flow chart illustrating an algorithm used in thisinvention to extend absolute pressure measurement range into thedifferential pressure sensor range, similar to FIG. 3, but with anadditional, low range absolute pressure sensor to extend the absolutepressure measurement range to even lower pressures as well;

FIG. 7 is a bifurcated bar chart similar to FIG. 4, but including thepressure measuring range of the second, lower, absolute pressure sensorfor producing absolute pressure measurements over the larger range.

FIG. 8 is a pressure profile of a conventional vacuum process cycleillustrating an example application of the integrated combinationabsolute and differential pressure transducer illustrated in FIGS. 6-7;and

FIG. 9 is a diagrammatic view of a vacuum process chamber similar toFIG. 1, but without the load lock, fitted with two absolute pressuresensors, one mid-range and the other low-range, and a differentialpressure sensor to extend the absolute pressure measuring range on acontinuous scale according to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is illustrated for example, but not forlimitation, in FIG. 1 by two pressure sensors 20, 30 connected in fluidflow relation to a chamber, which in this example is the interior 61 ofa load lock chamber 60, and a microprocessor 80, which receives andprocesses signals originating from the pressure sensors 20, 30 toprovide extended scale absolute pressure measurements of the chamberpressure P_(C), as represented by the display 90. The first pressuresensor 20 is an absolute pressure sensor (P_(ABS)) for sensing theabsolute pressure P_(C) in the chamber 61. The second pressure sensor 30is a differential pressure sensor (ΔP) for sensing the differencebetween the atmospheric pressure P_(A) outside the chamber 61 and thegas pressure P_(C) inside the chamber 61. In other words, P_(ABS)=P_(P),and ΔP=P_(A)−P_(P).

To provide some context to aid in understanding the invention, a loadlock 60 is often used in semiconductor fabrication to shuttle one ormore wafers 73 into and out of a vacuum process chamber 70, where one ormore feed gases delivered from feed gas sources 74, 75, 76 are reactedto deposit thin film materials, such as semiconductor material 77, onthe wafer 73. A vacuum pump 71 connected to the interior of the vacuumprocess chamber 70 pumps gases out of the vacuum process chamber 70 tomaintain a desired vacuum, i.e., low pressure, usually less than 1 torrand can be down to 10⁻⁸ torr or less, depending on the processrequirements. A platform 72 is usually provided in the vacuum processchamber 70 for supporting the wafer 73 during processing.

The interior chamber 61 of load lock 60 in this example is connected bya passage 69 to the interior of the vacuum process chamber 70. Thepurpose of the load lock 60 is to facilitate transfer of the wafers 73from the outside (e.g., ambient) atmosphere into the vacuum processchamber 70 without losing the vacuum in the vacuum process chamber 70 orallowing contaminants into the vacuum process chamber 70. Therefore, aninterior door or valve 62 in the passage 69 opens the passage 69 toallow transfer of wafers 73 into and out of the vacuum process chamber70 and closes the passage way 69 to seal the vacuum process chamber fromthe load lock chamber 61, when there are no such transfers beingconducted. In some process tools, there is an intermediate transferchamber (not shown) between the load lock and several process chambersto facilitate transferring wafers from one process chamber to anotherwithout having to go through the load lock or the atmosphere.

An exterior door 64 on the load lock 60 opens and closes the load lockchamber 61 to the atmosphere. When a wafer, illustrated in phantom lines73′ to indicate its transitory position, is being transferred from theoutside atmosphere into the load lock chamber 61, the interior door 62is closed, and the exterior door 64 is open. With the exterior door 64open, the absolute chamber pressure P_(C) in the load lock 60 issubstantially equal to the absolute atmospheric pressure P_(A) outsidethe chamber 61, as indicated at 91 in the display 90, so thedifferential pressure ΔP is zero. Meanwhile, the vacuum in the vacuumprocess chamber 70 behind the closed interior door 62 is maintained,i.e., at a much lower pressure P_(P) than the outside atmosphericpressure P_(A). Then, with the wafer 73′ inside the load lock chamber61, the exterior door 64 is closed, and a vacuum pump 65 connected tothe load lock chamber 61 pumps enough of the air and other gases out ofthe load lock chamber 61 to remove potential contaminants and to reducethe absolute chamber pressure P_(C), as indicated by the sloped line 92in the display 90, to a desired level 93 that substantially matches thelow absolute pressure P_(P) in the vacuum process chamber 70, which canbe measured with an absolute pressure sensor 78. At some chamberpressure P_(C), such as the pressure point 94, during the pump-downphase 92, where most of the air and potential contaminants have beenpumped out of the chamber 61, a common practice is to open a throttlevalve 66 to bolster the pump-down speed in the lower absolute chamberpressure P_(C) portion of the pump-down phase 92. This pressure point 94is often called a cross-over pressure, because it is where the pump-downof the chamber 91 crosses over from slow to high speed. However, thatterm for such operation or functionality is not used here to avoidconfusion with another cross-over function explained below, which ismore central to this invention.

With the absolute chamber pressure P_(C) pumped down to substantiallythe same level 93 as the absolute pressure P_(P) in the vacuum processchamber 70, the interior door 62 can be opened to allow the wafer 73 tobe transferred into the vacuum process chamber 70 and placed on theplatform 72. A moveable shuttle (not shown) in the load lock 60 is usedto move the wafer 72 into and out of the vacuum process chamber 70, asis well-known to persons skilled in the art.

With the wafer 73 in place on the platform 72, the interior door can beclosed for a time, while the semiconductor material 77 is deposited onthe substrate 73. The absolute chamber pressure P_(C) can be maintainedat the low level 93 during the deposition of the semiconductor material77. When the semiconductor material 77 is deposited on the wafer 73, theinterior door 82 is opened again to transfer the wafer 73 back into theload lock chamber 61. Then, the interior door 62 is closed again toisolate the interior of the vacuum process chamber 70 from the load lockchamber 61, so that the absolute chamber pressure P_(C) can be raised,as indicated at 95, back to atmospheric pressure P_(A), as indicated at96, without affecting the process pressure P_(P) in the vacuum processchamber 70. A common practice is to use a gas, such as nitrogen, or aninert gas, such as argon, from a source 63 to back-fill the load lockchamber 61 to raise the absolute chamber pressure P_(C) back up toatmospheric pressure P_(A), but air is also sometimes used for thispurpose.

While it is important to have accurate and reliable absolute pressuremeasurements of the chamber pressure P_(C) going down to the absolutechamber pressure P_(C) level 93 in order to match the process pressureP_(P) in the vacuum process chamber 70 before opening the interior door62, as explained above, it is also important to be able to measureaccurately when the chamber pressure P_(C) matches the atmosphericpressure P_(A) before the exterior door is opened.

Unfortunately, however, there are no absolute pressures sensors that canmeasure the absolute chamber pressure P_(C) accurately and reliably overthe full pressure range from atmospheric pressure P_(A) level 91 down tothe low pressure level 93 that is needed to match the process pressureP_(P) in the vacuum process chamber 70. Also, the atmospheric pressureP_(A) varies significantly with elevation above sea level and withambient weather conditions, so there is no fixed absolute pressure setpoint that can be used for the chamber pressure P_(C) to open theexterior door 64. Consequently, it is necessary to use an absolutepressure sensor 20 that is accurate and reliable at the lower pressurelevels to measure absolute chamber pressure P_(C) for determining whento open the interior door 62, but also to use a differential pressuresensor 30 for determining when the chamber pressure P_(C) matches theatmospheric pressure P_(A) in order to open the exterior door 64 at ornear atmospheric pressure P_(A) as indicated at 99 in the display 90. Ofcourse, two higher pressure range absolute pressure sensors (not shown),one for measuring absolute atmospheric pressure P_(A) and the other formeasuring absolute chamber pressure P_(C), and an analog or digitalcomparator circuit (not shown), microprocessor, or other means forcomparing such measurements, could be substituted for the differentialpressure sensor 30, as is understood by persons skilled in the art.Therefore, the use of the term “differential pressure sensor” herein ismeant to include not only conventional, direct read differentialpressure sensors or gauges, but also the use of two absolute pressuresensors with circuitry for subtracting measurements of one frommeasurements of the other to measure differential pressures, as well asany other apparatus or method that is capable of providing differentialpressure measurements.

Use of an absolute pressure sensor 20 for determining when the absolutechamber pressure P_(C) is low enough to open the interior door 62 incombination with a differential pressure sensor 30 for determining whento open the exterior door 64 of a load lock 60 is well-known, asexplained in the published U.S. patent application Ser. No. 09/907,541(now U.S. Pat. No. 6,672,171) and U.S. patent application Ser. No.09/815,376 (now U.S. Pat. No. 7,076,920), both of which are incorporatedherein by reference. However, as mentioned above, many vacuum processchamber operators, quality control personnel, and others would like tohave a full absolute pressure profile 98 on a single scale that extendsover the full chamber pressure P_(C) range of operation from absoluteatmospheric pressure P_(A) level 91, 96 or above, down to the lowestabsolute pressure level 93 or below for diagnostic, quality control,design, maintenance, and other reasons.

An important feature of this invention, therefore, is to integratepressure measurements from at least one low or mid-level absolutepressure sensor 20 with differential pressure measurements from thedifferential pressure sensor 30 to produce accurate and reliableabsolute pressure measurements of the chamber pressure P_(C) on a singlescale that extends over a range low enough for opening the interior door62 in this kind of load lock as well as other process applications andhigh enough to provide a complete absolute chamber pressure P_(C)profile 98, including absolute atmospheric pressure P_(A). However, thisinvention is not limited to load lock applications. On the contrary, itis useable for any other application in which such an extended absolutepressure measurement range beyond (above or below) the acceptableaccuracy and reliability range of a low or mid-level absolute pressuresensor is needed or desired.

According to this invention, as illustrated in FIG. 2, the measurementsprovided by the absolute pressure sensor 20 are used for the absolutechamber pressures P_(C) below a cross-over pressure P_(X) level or rangeon an absolute pressure scale 40 in the display 90, where the absolutepressure sensor 20 has the capability to provide accurate and reliableabsolute pressure measurements. However, for absolute chamber pressureP_(C) measurements above the cross-over pressure level or range P_(X),where the absolute pressure sensor 20 does not provide sufficientlyaccurate and reliable absolute pressure measurements, normalized virtualdifferential pressure measurements, which are based on differentialpressure measurements P₃₀ from the differential pressure sensor 30 areused for the absolute chamber pressure P_(C) on the absolute pressurescale 40 in the display 90. To normalize such differential pressuremeasurements P₃₀ from the differential pressure sensor 30 for thispurpose, they are correlated with measurements P₂₀ of absolute chamberpressure P_(C) from the absolute pressure sensor 20 at or below acorrelation pressure threshold point P_(t), which is preferably at ornear a pressure where the practical accuracy of the differentialpressure sensor 30 effectively reaches its bottom, i.e., where furtherdecimal places of pressure measurements are not meaningful orsignificant in a practical application or where physical structure orelectric circuit limitations render lower differential pressure ΔPmeasurements with the differential pressure sensor 30 practicallymeaningless. This correlation pressure threshold point P_(t), or anypressure lower than P_(t) that is still within an accurate and reliableabsolute pressure measuring range of the absolute pressure sensor 20,can be used as a base line for adjusting or normalizing the differentialpressure ΔP measurements P₃₀ from the differential pressure sensor 30 tocorrelate with the absolute chamber pressure P_(C) measurements P₂₀ fromthe absolute pressure sensor 20 at a desired level of accuracy, as willbe explained in more detail below. Therefore, such adjustment ornormalizing correlation factor F can be used to convert or normalize allhigher differential pressure ΔP measurements P₃₀ from the differentialpressure sensor 30 to the same scale as the absolute chamber pressureP_(C) measurements P₂₀ from the absolute pressure sensor 20, as is shownin FIG. 3 and will be described in more detail below. Consequently, asthe chamber pressure P_(C) rises above that correlation pressurethreshold P_(t), the correlation factor F can be added to all of thedifferential pressure ΔP measurements from the differential pressuresensor 30 to convert them to virtual absolute chamber pressure P_(C)measurements P_(V) correlated to the same absolute pressure scale 40 asthe absolute pressure measurements P₂₀ produced by the absolute pressuresensor 20.

Eventually, as the chamber pressure P_(C) continues to increase, it willreach some chamber pressure P_(C) level that is still substantiallybelow the atmospheric pressure P_(A), but which is above the accurateand reliable absolute pressure measuring capability of the absolutepressure sensor 20. Therefore, the absolute chamber pressure P_(C)measurements P₂₀ by the absolute pressure sensor 20 above that accuracyand reliability level become unreliable and unusable. However, thedifferential pressure sensor 30 continues to provide accurate andreliable differential pressure ΔP measurements as the chamber pressureP_(C) rises all the way up to the atmospheric pressure P_(A) and beyond.Therefore, in the higher chamber pressure P_(C) levels, where theabsolute pressure measurements P₂₀ by the absolute pressure sensor P₂₀are unreliable, accurate and reliable virtual absolute chamber pressureP_(C) measurements P_(V) can be provided on the same continuous scale 40all the way up to the atmospheric pressure P_(A) level and beyond byadding the correlation factor F to the differential pressure ΔPmeasurements P₃₀ from the differential pressure sensor 30.

It is preferable, however, to not wait until the absolute pressuresensor 20 reaches the end of its accuracy and reliability range beforecrossing over to the virtual absolute chamber pressure P_(C)measurements P_(V) for output by the microprocessor 80 and display bythe display device 90. Instead, it is preferable, but not essential, toselect a cross-over pressure level P_(X), which can be either a distinctcross-over pressure point or a cross-over pressure range with asmoothing function (explained in more detail below), at which thedisplayed absolute chamber pressure profile 98 is assembled from thevirtual absolute chamber pressure P_(C) measurements P_(V) rather thanfrom the absolute pressure sensor 20 measurements P₂₀, as will beexplained in more detail below.

To further illustrate one of the principles on which this invention isfounded, the absolute pressures P_(ABS) in a typical vacuum processrange are juxtaposed in FIG. 2 to corresponding differential pressuresΔP on a logarithmic scale 40 in units of torr, although any other unitsof pressure could also be used. In the first column 42 in FIG. 2, ascale of absolute pressures P_(ABS) is shown extending from 1,000 torrat the upper end of the scale 40 down to 10⁻⁵ torr (0.00001 torr) at thelower end of the scale 40. Some processes are performed in pressures aslow as 10⁻⁸ torr or lower, but it is not necessary to extend the scale40 in this FIG. 2 that low in order to convey the principles of thisinvention.

Two arbitrary different example atmospheric pressures P_(A)—sea leveland high plains, e.g., Boulder, Colo. U.S.A.—are used in FIG. 2 tofacilitate an explanation of this invention, but any other exampleatmospheric pressures could also be used. Also, while 760 torr isconsidered by persons skilled in the art to be a standard atmosphericpressure P_(A) at sea level, the actual atmospheric pressure P_(A) willvary above and below that 760 torr level due to various weatherpatterns. Likewise, while 630 torr is a common atmospheric pressureP_(A) at high plains and foothills locations, such as Boulder, Colo.U.S.A., the actual atmospheric pressure P_(A) at any such location willvary above and below that level as weather patterns change. However,such variations are accommodated by this invention in order to continuemaking very accurate and reliable absolute pressure P_(ABS)measurements, regardless of atmospheric pressure P_(A) changes, as willbe understood by persons skilled in the art, once they understand thisinvention. In fact, as mentioned above, one of the purposes of thisinvention is to provide accurate and reliable chamber pressure P_(C)measurements up to and beyond whatever local atmospheric pressure P_(A)might exist at any time, even as such local atmospheric pressure P_(A)changes from day-to-day, hour-to-hour, or smaller increments of time.

As illustrated in FIG. 2, when the atmospheric pressure P_(A) is, forexample, 760 torr and the absolute chamber pressure P_(C) (FIG. 1) isalso 760 torr, such as when the exterior door 64 is open, then thedifferential pressure ΔP between the atmospheric pressure P_(A) and thechamber pressure P_(C) in the chamber 61 (FIG. 1) will be, of course,zero, as shown in column 44 in FIG. 2. As the chamber 61 is evacuated bythe vacuum pump 65 (FIG. 1), and the absolute chamber pressure P_(C) islowered by 660 torr to the absolute chamber pressure P_(C) of, forexample, 100 torr, the differential pressure ΔP will also drop from zeroto −660 torr, shown in FIG. 2.

Likewise, if the process is being performed at a high plains locationwith an atmospheric pressure P_(A) of, for example, 630 torr instead ofthe 760 torr sea level atmospheric pressure, the differential pressureΔP between the atmospheric pressure P_(A) and the chamber pressure P_(C)with the exterior door 64 open, will also be zero, as shown in FIG. 2.However, there is a 130 torr difference between the 760 torr sea levelP_(A) and the 630 torr high plains P_(A). Therefore, as the chamber 61is evacuated down to an absolute chamber pressure P_(C) of 100 torr, thedifferential pressure ΔP at the high plains location will be −530 torrinstead of the −660 torr at the sea level location.

The same 130 torr discrepancy between this sea level example and thishigh plains location example shows in all of the differential pressureΔP measurements as the chamber pressure P_(C) is lowered. In otherwords, lowering the absolute chamber pressure P_(C) of 100 torr down to10 torr—a 90 torr drop—correspondingly lowers both the sea levellocation differential pressure ΔP and the high plains locationdifferential pressure ΔP by 90 torr, i.e., from −660 torr down to −750torr at the sea level location and −530 torr at the high plainslocation, as shown in FIG. 2. Likewise, lowering the absolute chamberpressure P_(C) another 9 torr down to 1 torr would lower thedifferential pressures ΔP at the sea level location and the high plainslocations to −759 torr and −629 torr, respectively. An absolute chamberpressure P_(C) of 0.1 torr corresponds to differential pressures of ΔPof −759.9 torr at the sea level location and −629.9 torr at the highplains location, and 0.01 torr, 0.001 torr, 0.0001 torr, and 0.00001torr absolute chamber pressures P_(C) correspond to −759.99 torr,−759.999 torr, −759.9999 torr, and −759.99999 torr at the sea levellocation and to −629.99 torr, −629.999 torr, −629.9999 torr, and−629.99999 torr at the high plains location.

While there are absolute pressure sensors available that can measurepressures down to 0.00001 torr (i.e., 10⁻⁵ torr) and below quiteaccurately and reliably, the available differential pressure sensorseffectively bottom out in accuracy at about one decimal place, i.e.,when the absolute chamber pressure P_(C) is about 0.1 torr. At aboutthat level, differential pressure ΔP measurements down to any furtherdecimal places, such as −759.99 torr or −759.999 torr for the examplesea level location and −629.99 torr or −629.999 torr for the examplehigh plains location, are practically meaningless, just as thedifference between differential pressure ΔP measurement P₃₀ with thedifferential pressure sensor 30 of −660.99 torr and −660.999 torr at thesea level location would be practically meaningless, because theavailable differential pressure sensors are just not accurate to morethan one or two decimal places in torr units. However, in the upperpressure ranges, for example, above 100 torr, measurement accuracies toone or even zero decimal places is usually sufficient for most processoperations. Therefore, correlating absolute chamber pressure P_(C)measurements P₂₀ from the absolute pressure sensor 20 to the effectivelybottomed-out differential pressure ΔP measurements P₃₀ from thedifferential pressure sensor 30 when the absolute chamber pressure P_(C)is below a threshold pressure P_(t) of 1 torr or perhaps 0.1 torr orlower, depending on the decimal places of accuracy needed at higherdifferential pressure ΔP measurements, provides an effective, accurate,and repeatable baseline for normalizing the differential pressure sensor30 measurements to the absolute pressure sensor 20 measurements. Withsuch baseline normalization, the differential pressure sensor 30measurements P₃₀ can, according to this invention, be normalized orconverted with a normalizing correlation factor to virtual absolutechamber pressure P_(C) measurements P_(V), including in the higherchamber pressure P_(C) ranges, where the differential pressure sensor 30is most accurate and dependable and the absolute pressure sensor 20losses its accuracy and dependability. For example, if the differentialpressure ΔP measurements P₃₀ of differential pressure sensor 30 arenormalized to the accurate and reliable absolute chamber pressure sensor20 measurements P₂₀, when the chamber pressure P_(C) is at a baselinethreshold pressure P_(t) of 1 torr or below, then, as the absolutechamber pressure P_(C) rises above some cross-over pressure level P_(X),such as 100 torr, where the differential pressure sensor 30 is moreaccurate and reliable than the absolute pressure sensor 20, thedifferential pressure ΔP measurements P₃₀ of the differential pressuresensor 30 can be converted to accurate and reliable virtual absolutechamber pressure P_(C) measurements P_(V) all the way up to atmosphericpressure P_(A) level and above. In fact, a piezo differential pressuresensor is accurate from about 100 torr below atmospheric pressure P_(A)to about 1,500 torr above atmospheric pressure P_(A), which is a rangeof about 1,600 torr. Therefore, normalizing such differential pressuremeasurements P₃₀, as described above, can provide accurate anddependable virtual absolute pressure measurements P_(V) over a 1,600torr range. The exact lower and upper ends 43, 45 of such virtualabsolute pressure measurement P_(V) range on the absolute pressure scale40 will depend on what the atmospheric pressure P_(A) happens to be atany particular time, as illustrated in FIG. 4, which is described inmore detail below.

An example normalizing method for implementing this invention to producesuch virtual absolute chamber pressure P_(C) measurements P_(V) isillustrated in FIGS. 3 and 4, although other methods or variations canbe devised by persons skilled in the art, once they understand theprinciples of this invention. For purposes of explanation of this methodin FIG. 3, reference is also made to a chart of effective measuringranges of example absolute and differential pressure sensor 20, 30ranges in FIG. 4 and an example load lock chamber 61 pressure profile inFIG. 5, as well as with continuing reference to the FIGS. 1 and 2discussed above.

In this example, the absolute pressure sensor 20 is shown in FIG. 4 tohave an effective measuring range extending from about 100 torr down toabout 10⁻⁵ torr, and the absolute pressure measurements by the absolutepressure sensor 20 are called P₂₀ for convenience. The micropiraniabsolute pressure sensor described in U.S. patent application Ser. No.09/907,541, (now U.S. Pat. No. 6,672,171), which is incorporated hereinby reference, has this kind of range, while conventional convectionpirani absolute pressure sensors have a somewhat narrower range of about100 torr down to about 10⁻³ torr. Other thermal conductivity typeabsolute pressure sensors as well as diaphragm based absolute pressuresensors, such as low range capacitive manometers, low range piezo,strain gauge, and others also are effective in the upper portions ofthis example absolute pressure sensor range.

The differential pressure sensor 30 for this example description isshown in FIG. 4 to have an effective measuring range from aboveatmospheric pressure (e.g., about 1,500 torr above P_(A)) down to aboutminus 99.9 percent of atmospheric pressure (−99.9% Atm), i.e., down toabout 10⁻¹ torr, although it is more accurate and reliable above about 1torr. The differential pressure measurements by the differentialpressure sensor 30 are called P₃₀ for convenience, and, as mentionedabove, absolute pressure measurements by the absolute pressure sensor 20are called P₂₀.

The bar chart in FIG. 4 is divided into a differential pressure domain42 on top and an absolute pressure domain 44 on bottom to aid inillustrating the concept that the differential pressure sensor 30 outputmeasurements P₃₀ shift laterally in relation to the absolute pressurescale 40 and to the absolute measurements P₂₀, as indicated by arrow 41and phantom lines 43, 45 in FIG. 4, with the amount of such shiftdepending on the changes in atmospheric pressure. If the atmosphericpressure P_(A) is higher, the differential pressure measurements P₃₀shift to the right in relation to the absolute pressure seal 40 in FIG.4, and, if the atmospheric pressure P_(A) moves lower, the differentialpressure measurements shift to the left in relation to the absolutepressure scale 40.

The absolute pressure profile 98 in FIG. 5 is an enlargement of theabsolute pressure profile 98 for a typical load lock cycle in thedisplay 90 of FIG. 1. The scale 40 at the left is absolute pressure intorr, the scale 140 at the right is differential pressure in torr, andthe scale 142 at the bottom is time in minutes, although any suitablepressure and time units can be used. To re-cap, the cycle starts atatmospheric pressure P_(A) indicated at 91, and it descends downwardly92 during evacuation of the load lock chamber 61 to a base levelpressure 93, where it is held for a time during transfer of a wafer 73into (and, if desired, back out of) the reaction chamber 70 (FIG. 1).Then, as the load lock chamber 61 (FIG. 1) is back-filled, the absolutechamber pressure P_(C) rises back, as shown at 95, to atmosphericpressure P_(A) as indicated at 96.

The absolute chamber pressures P_(C) for the absolute pressure profile98, as well as for certain switching functions for the load lockoperation (e.g., opening the throttle valve 66 at a pressure point 94,and opening the interior door 62 at a pressure point 97) can beprovided, for example, by the method 10 shown in FIG. 3. At the start ofthe method 10, it is desirable, although not necessary, to choose aninitial correlation factor F₀ 46 for use in converting differentialpressure measurements P₃₀ to normalized virtual absolute pressuremeasurements P_(V). For example, the initial correlation factor F₀ couldbe at 760 torr for a sea level location or at 630 torr at a high plainslocation, which would provide an approximation good enough for the firstpart of the absolute pressure profile 98 down to the cross-over pressureP_(X) for the first load lock cycle. Therefore, the correlation factor Fis set at 47 in FIG. 3 equal to the initial correlation factor F₀. Theabsolute pressure measurement P₂₀ and the differential pressuremeasurement P₃₀ are read at 48 from the absolute pressure sensor 20 andfrom the differential pressure sensor 30.

The differential pressure ΔP is set at 49 to the value of themeasurement P₃₀ and is output at 50 for any desired display or controlfunctions, such as to open the exterior door 64 of the load lock 60(FIG. 1) when ΔP=0, as indicated at 99 in the example process pressureprofile in FIG. 5. Then, the virtual absolute chamber pressuremeasurement P_(V) is calculated at 51 by adding the correlation factor Fto the differential pressure measurement P₃₀. For example, if thecorrelation factor F is 760 torr, then the virtual absolute chamberpressure measurement P_(V) is the differential pressure measurement P₃₀plus 760 torr.

Next, as shown in FIG. 3, if the absolute chamber pressure P_(C)measurement P₂₀ is not greater than the cross-over pressure level P_(X)at 52, then the absolute chamber pressure P_(C) is set equal to theabsolute pressure measurement P₂₀ at 53. If P₂₀ is greater than thecross-over pressure level P_(X) at 52, then the absolute chamberpressure P_(C) is set equal to the virtual absolute chamber pressuremeasurement P_(V) at 54. Then, the absolute chamber pressure P_(C),whether set equal to P₂₀ at 53 or equal to P_(V) at 54, is output at 55for any desired control and display functions. Alternatively, the testat 52 could be whether P_(V)>P_(X) instead of whether P₂₀>P_(X), with anequivalent effect.

Finally, if the absolute chamber pressure measurement P₂₀ is not lessthan the correlation threshold pressure P_(t) at 56, then the method 10loops back via 57 to obtain another reading of the absolute chamberpressure measurement P₂₀ and of the differential pressure measurementP₃₀ for another iteration through the logic to obtain new ΔP and P_(C)values as the chamber pressure P_(C) decreases. However, if the absolutechamber pressure measurement P₂₀ is less than the correlation thresholdpressure P_(t) at 56, then the correlation factor F is recalculated at58 before the method 10 loops back via 59 for another iteration. Asdiscussed above the correlation threshold pressure P_(t) is preferably,but not necessarily, low enough to be at or near the bottom of thedifferential pressure measuring capability of the differential pressuresensor 30 at whatever minimum decimal place is needed for the precisiondesired in the virtual pressure measurements P_(V). However, P_(t)should not be so low that the actual absolute chamber pressure P_(C)never or rarely gets that low, because, according to the method 10, thecorrelation factor F only gets updated to compensate for any atmosphericpressure P_(A) changes due to weather changes or other causes, when theabsolute chamber pressure P_(C) falls below P_(t). Therefore, if thechamber pressure P_(C) is cycled to drop below P_(t) every hour, forexample, the correlation factor F will be updated every hour tocompensate for any atmospheric pressure P_(A) changes due to weather orotherwise. Such updates keep the P_(C) outputs at 55 accurate, evenabove P_(X), regardless of atmospheric pressure P_(A) changes.

While setting the correlation pressure threshold P_(t) at or very near alevel where the differential pressure sensor 30 measuring capabilitiesessentially zero out is very convenient and effective, especially forprocesses that cycle down below that level quite often, any pressurelevel where the relationship between an accurate, reliable, absolutepressure measurement and a differential pressure is known can be used todetermine the correlation factor. For example, if the absoluteatmospheric pressure P_(A) is known from some other source at aparticular instant in time when the differential pressure sensormeasures zero, such absolute atmospheric pressure P_(A) value can beused to set the correlation factor F. Such other source could be, forexample, another absolute pressure sensor that is accurate and reliableat that level.

As mentioned above, and as shown in FIG. 4, the absolute chamberpressure P_(C) output at 55 in the method 10 in FIG. 3 comprises acontinuous composite of absolute pressure measurements P₂₀ and virtualabsolute pressure measurements P_(V), which effectively extends therange of accurate and reliable absolute chamber pressure P_(C) measuringcapability higher than the capability of the absolute pressure sensor 20alone—all the way up to atmospheric pressure P_(A) and beyond. Thecross-over pressure P_(X) is preferably selected to be at the level atwhich virtual absolute pressure measurements P_(V) are more accurate andreliable than absolute pressure measurements P₂₀ from absolute pressuresensor 20 and vice-versa. This simple cross-over at P_(X), where P_(X)is a single pressure point, is acceptable for many applications.However, any common smoothing function, which is known to personsskilled in the art, can be used, if desired, to blur or spread thecross-over level over a range 103 to ensure that the pressure profile 98in FIG. 5 does not have a sharp bend or kink where the cross-overoccurs. Basically, a smoothing function starts by weighting the P_(C)value more with P₂₀ at one end of the range 103 and changing graduallyto weighting the P_(C) value more heavily with P_(V) at the other end ofthe range 103 so that the P_(C) output in the range 103 would be ablended value of P₂₀ and P_(V).

With the correlation factor F updated at 58 in FIG. 3 after the absolutechamber pressure measurement P₂₀ falls below the correlation thresholdpressure P_(t), as explained above, the output P_(C) at 55 is anaccurate measurement of the absolute chamber pressure P_(C) all the wayup the rising portion 95 of the chamber pressure profile 98 toatmospheric pressure 96 and above. However, the differential pressure ΔPoutput at 50 in FIG. 3 can still be used to open the exterior door 64(FIG. 1), if desired. That same updated correlation factor F continuesto be used for the correlation of the differential pressure measurementsP₃₀ to virtual absolute pressure pressure measurements P_(V) at 49 inFIG. 3 for the descending portion 92 (FIG. 5) of the next load lockevacuation cycle, since it is the correlation factor F that is relatedmost recently to the actual atmospheric pressure P_(A) and is,therefore, usually more accurate than the initial correlation factor F₀.

The logic in the process discussed above and illustrated in FIGS. 3-5can be implemented in any manner, such as a microprocessor 80, as shownin FIG. 1 or any other convenient manner known to persons skilled in theart. Signals from the absolute and differential pressure sensors 20, 30can be communicated to the microprocessor 80 by any convenientcommunication links 21, 31, and signals from the microprocessor tooperate the doors 62, 64, the throttle valve 66, and the display 90 canbe via suitable communication links 84, 83, 68, 86, respectively. Suchcommunications links can be hard wired, radio frequency, infrared,sound, or any other signal communication technique. Also, the signalsand processed information can be handled or stored in any buffers,filters, amplifiers, analog-to-digital converters, memory devices, andother conventional signal processing components (not shown), as is knownto persons skilled in the art. The display 90 is used generically hereand can be visual, printed, projected, or any component for receiving,using, storing, or displaying the pressure outputs ΔP and/or P_(C) fromthe process described in relation to FIG. 3 or otherwise in accordancewith this invention. The pressure sensor 78 can be, but does not have tobe, connected to the microprocessor 80 via communication link 85 formonitoring, comparison, display, or the like.

As explained above and illustrated in FIG. 4, the absolute pressuresensor 20, with current technologies, is unlikely to be able to measurevery low absolute pressures, e.g., below 10⁻⁴ or 10⁻⁵, and still extendhigh enough, e.g., 1 to 100 torr, to get within a useable correlationrange, e.g., 10⁻² to 1 torr. Absolute pressure sensors 20 in theseranges are considered to be mid-range absolute pressure sensors. If itis desired to evacuate the chamber pressure P_(C) down to even lowerpressure levels, such as the 10⁻⁷ torr process base pressure level 113in the example process pressure profile 98 in FIG. 5, a second, lowabsolute pressure sensor 25, as shown in FIGS. 8 and 9 can be added tothis invention. The low absolute pressure sensor 25 for example, an iongauge, a hot cathode pressure sensor, or a cold cathode pressure sensor,can extend the range of accurate and reliable absolute chamber pressureP_(C) outputs down to 10⁻⁸ torr or below, as illustrated in FIG. 7. Anion gauge, a hot cathode gauge, and a cold cathode gauge are examples ofabsolute pressure sensors that are capable of providing accurate andreliable absolute pressure measurements P₂₅ at these low absolutepressure levels.

To illustrate the operation of this second, lower range, absolutepressure sensor 25 in combination with the mid-range absolute pressuresensor 20 and the differential pressure sensor 30 in this invention,reference is made now primarily to FIGS. 6-9. In this example, theprocess chamber 70, shown in FIG. 9, is not equipped with a load lock,and the exterior door 164 is positioned to open and close the passage 69into the process chamber 70. The two absolute pressure sensors 20, 25and the differential pressure sensor 70 are all connected directly influid flow relation to the interior 161 of the chamber 70. The chamberpressure P_(C) in this example, therefore, is the pressure in theinterior 161 of the process chamber 70. The pressure scales 40, 140 andtime scale 142 in FIG. 8 are similar to those in FIG. 5.

The mid-range absolute pressure sensor 20 and the differential pressuresensor 30 are correlated together in substantially the same manner asdescribed above for the FIGS. 1-5 example. The initialization of thecorrelation factor F with an initial F₀ can be the same, as shown inFIG. 6. All three sensors 20, 25, 30 are read at 48 in FIG. 6 for thetwo absolute pressure measurements P₂₀, P₂₅ and for the differentialpressure measurement P₃₀. When a process, such as that illustrated bythe pressure profile 110 in FIG. 8, starts with the chamber pressureP_(C) equal to atmospheric pressure P_(A), when the door 164 is closed,the differential pressure sensor 30 is the only one that outputsaccurate and reliable measurements at that pressure level 112 and in theinitial portion 114 of the chamber pressure P_(C) descent. Therefore,the differential pressure ΔP is the same as the differential pressuremeasurement P₃₀, as shown at 49, and it is output at 50 for whateverfunctionalities are desired, such as displays of differential pressureΔP on a differential pressure scale 140 and opening the door at 132after the process is completed. However, as explained above, adjustingthe differential pressure measurement P₃₀ with the correlation factor Fat 51 in FIG. 6 also produces a virtual absolute chamber pressuremeasurement P_(V), which is as accurate as the correlation factor F, andwhich is output as the absolute chamber pressure P_(C) at 55 for thatinitial portion 114 down to the cross-over pressure P_(X). Then, whenthe chamber pressure P_(C) is lowered to a range in which mid-rangeabsolute pressure sensor 20 provides more accurate and reliable pressuremeasurements P₂₀, for example, in the range 116 below the firstcross-over pressure P_(X) in FIG. 8, the chamber pressure P_(C) outputat 55 in FIG. 6 is equal to the absolute pressure measurements P₂₀. Apressure point 94 in this mid-range 116 can be used to open the throttlevalve 66 in FIG. 9 to bolster pumpdown speed, as explained for the FIGS.1-5 example above. Also, as explained above, when the chamber pressureP_(C) drops below the correlation pressure threshold P_(t), thecorrelation factor F is updated at 58 in FIG. 6 to compensate for anychange in the atmospheric pressure P_(A) that may have occurred sincethe previous update of the correlation factor F. The connections, signalcommunications links, microprocessor 80, and other signal processing andhandling components in the example of FIG. 9 can be similar to thosedescribed for FIG. 1, as would be obvious to persons skilled in the art,once they understand the principles of this invention, thus need not bedescribed further herein.

Unlike the example process pressure profile 98 in FIG. 5, however, theexample process pressure profile 110 illustrated in FIG. 8 continuesbelow the pressure measuring capability of the mid-range pressure sensor20. Therefore, from a second cross-over pressure level P_(XX) or range118, preferably where the pressure measurements P₂₀, P₂₅ are both stillaccurate and reliable, and down through a lower portion 120 of theprocess pressure profile 110 under P_(XX) to the base pressure 122, theabsolute chamber pressure P_(C) output at 55 in FIG. 6 is equal to theabsolute pressure measurement P₂₅ from the low range pressure sensor 25.This second cross-over at P_(XX) is implemented at 124 and 126 in FIG.6.

The base pressure level 122 is commonly used to draw as many impuritiesout of the process chamber 161 as practically possible beforeback-filling the process chamber 161 with an inert or an overpressuregas 63 to raise the process chamber pressure P_(C) to a more mid-levelpressure level 126, where the process feed gases 74, 75, 76 are flowedinto the process chamber 161 to react and deposit the semiconductormaterial 77 on the substrate 73. The process pressure level 126 can beabove, below, or equal to the second cross-over pressure P_(XX), asdesired by an operator.

At the completion of the example semiconductor 77 deposition process inFIGS. 6-9, the feed gasses 74, 75, 76 are turned off, and the back-fillgas 63 or another back-fill gas (not shown) is used to raise the processchamber pressure P_(C) back up to the atmospheric pressure P_(A).Through the mid-range 128 between the second cross-over pressure P_(XX)and the first cross-over pressure P_(X), the absolute pressure of theprofile 110 is provided by the absolute pressure measurements P₂₀.Finally, in the pressure range 130 above the first cross-over pressureP_(X), the absolute pressure P_(C) measurements for the process pressureprofile 110 are again provided by the virtual absolute pressuremeasurements P_(V), which are calculated by adding the updatedcorrelation factor F to the differential pressure measurement P₃₀ asexplained above and shown at 52, 54, 55 of FIG. 6. Finally, when thechamber pressure P_(C) reaches atmospheric pressure P_(A), thedifferential pressure sensor 30 senses zero differential pressure ΔP,and the ΔP output at 53 can be used to open the door 164 at 132, asexplained above.

As explained above, any pressure level at which an accurate relationshipbetween absolute pressure and differential pressure is known or can bemeasured or otherwise determined can be used to determine a correlationfactor and, with the differential pressure measurements, to extendabsolute pressure measurements beyond the accurate and reliable absolutepressure measuring capabilities of an absolute pressure sensor. Theexamples described above show this invention extending the range ofabsolute pressure measurements above the accurate and reliable absolutepressure measuring capabilities of an absolute pressure sensor by addingthe correlation factor F to the differential pressure measurements P₃₀.However, the principles of this invention also work for determining andusing a correlation factor along with differential pressure measurementsto extend absolute pressure measurements below the accurate anddependable pressure measuring capabilities of a high range absolutepressure sensor. For example, if an absolute pressure sensor (not shown)is capable of measuring high absolute pressures, such as from 500 to3,000 torr accurately and reliably, but could not measure absolutepressures below 1,000 torr, a differential pressure sensor that isaccurate and reliable from +200% atmosphere, down to −99.9% atmosphere(approximately 1,200 to 1,500 torr down to about −760 to −600 torr,depending on the specific atmospheric pressure at the time) along withan appropriate correlation factor could be used to extend the absolutepressure measuring range below the 1,000 torr low range limit of theabsolute pressure sensor, e.g., down to 0.1 torr. The correlation factorcould be determined, for example, at atmospheric pressure, where thedifferential pressure is zero. If desired, the absolute pressuremeasurements could then be extended down to even lower pressures, e.g.,below 1 torr down to 10⁻⁸ torr, by combining a mid-range absolutepressure sensor and a low-range absolute pressure sensor, as explainedabove.

Consequently, a differential pressure sensor can, according to thisinvention, be used to provide virtual absolute pressure measurementsP_(V) within its accurate and reliable differential pressure measuringrange, on a common absolute pressure scale with absolute pressuremeasurements of one or more absolute pressure sensors above and/or belowthe differential pressure sensor range. This capability is advantageous,even if absolute pressure sensors with accurate and reliable pressuremeasuring capabilities are available for the same range as thedifferential pressure sensor in some circumstances. For example, in theFIG. 4 example described above, the absolute pressure sensor 20 could bea micropirani sensor, which, like a number of other absolute pressuresensors, a thermal conductivity type pressure sensor. Pressure readingsfrom thermal conductivity pressure sensors change with different kindsof gasses, i.e., with different molecular contents, at higher pressures,such as over about 1 torr. In other words, for example, a thermalconductivity type absolute pressure sensor will output differentpressure readings P₂₀ when the gas in the chamber is changed or mixedwith another gas, even if the actual absolute pressure P_(C) in thechamber does not change. Direct reading differential pressure sensors,such as piezo and capacitance diaphragm gauges, are not gas-typedependent, thus provide the same pressure readings regardless of thekinds of gasses that are introduced into the chamber. Therefore, forgas-independent absolute pressure measurements, use of the differentialpressure measurements P₃₀ with a correlation factor F according to thisinvention has advantages over a thermal conductivity absolute pressuresensor for the same range. Therefore, the absolute pressure sensor notonly performs two functions simultaneously according to this invention,i.e., measuring and monitoring both differential and absolute pressuresin a range of about 10 torr to 1,500 torr or higher, it can also providethe absolute pressure readings in that range better than at least someabsolute pressure sensors that are gas-type dependent.

The foregoing description is considered as illustrative of theprinciples of the invention. Furthermore, since numerous modificationsand changes will readily occur to those skilled in the art, it is notdesired to limit the invention to the exact construction and processshown and described above. Accordingly, resort may be made to allsuitable modifications and equivalents that fall within the scope of theinvention. For example, while the comparisons in decision boxes 50, 56in FIGS. 3 and 6 are expressed in terms of “greater than” and “lessthan”, they could be “greater than or equal to” or “less than or equalto”, respectively, because these pressure levels PX and Pt do not haveto be exact for this invention to function properly. Therefore, theterms “greater than” (>) is considered to include “greater than or equalto” ( ), and the term “less than” is considered to include “less than orequal to” ( ), for the explanations or recitations relating to themethods in FIGS. 3 and 6. Also, in both FIGS. 3 and 6, the comparison at52 can be “Is P_(V)>P_(X)?” instead of “Is P₂₀>P_(X)?”, as indicatedabove. As mentioned above, example current technology absolute pressuresensors suitable for use in this invention include the thermalconductivity-type sensors, micropirani, conventional convention pirani,hot cathode, cold cathode, ion gauge, etc., as well as low-rangediaphragm sensors such as capacitive, piezo, strain gauge, and the like.Any diaphragm differential pressure sensor and combinations of absolutepressure sensors, as mentioned above, are suitable for use in thisinvention. Of course, the invention would also work with any futuretechnology absolute or differential pressure sensors. The words“comprise,” “comprises,” “comprising,” “include,” “including,” and“includes” when used in this specification are intended to specify thepresence of stated features, integers, components, or steps, but they donot preclude the presence or addition of one or more other features,integers, components, steps, or groups thereof.

1. A method of providing an absolute chamber pressure profile of gaspressure in a chamber as the pressure in the chamber changes over time,comprising: measuring absolute pressure in the chamber with an absolutepressure sensor that has an accurate and dependable range of absolutepressure measuring capability; measuring differential pressure betweenthe chamber and atmospheric pressure with a differential pressure sensorthat has an accurate and dependable range of differential pressuremeasuring capability, which corresponds with at least a portion of theaccurate and dependable range of the absolute pressure sensor;determining a correlation factor between an absolute pressuremeasurement from the absolute pressure sensor that is considered to beaccurate and reliable and a differential pressure measurement from thedifferential pressure sensor; using the absolute pressure measurementsfrom the absolute pressure sensor for the absolute chamber pressureprofile in pressure ranges where the absolute pressure measurements fromthe absolute pressure sensors are more accurate and reliable than thedifferential pressure measurements from the differential pressuresensor; using the differential pressure measurements from thedifferential pressure sensor, adjusted by the correlation factor toprovide virtual absolute pressure measurements, for the absolute chamberpressure profile where the differential pressure measurements from thedifferential pressure sensor are more accurate and reliable than theabsolute pressure measurements from the absolute pressure sensor; andreceiving, using, storing, or displaying the absolute pressure profilein a display.
 2. The method of claim 1, including determining thecorrelation factor by: measuring the absolute pressure in the chamber ata chamber pressure; measuring the differential pressure between theatmosphere and the chamber at said chamber pressure; and subtracting thedifferential pressure measurement at said chamber pressure from theabsolute pressure measurement at said chamber pressure.
 3. A method formeasuring absolute pressures in a chamber over a variable pressure rangecomprising measuring the absolute pressures in the chamber on anabsolute pressure scale over a first portion of the variable pressurerange with an absolute pressure gauge; measuring the differentialpressure between the chamber and the atmosphere on a differentialpressure scale over a second portion of the variable pressure range thatpartially overlaps the first portion of the variable pressure range;subtracting a differential pressure measurement measured at a particularchamber pressure from an absolute pressure measurement measured at saidparticular chamber pressure to determine a correlation factor;normalizing the differential pressure measurements in at least some ofthe second portion of the variable pressure range to said absolutepressure scale by adjusting said differential pressure measurements inat least some of the second portion of the variable pressure range withthe correlation factor; and receiving, using, storing, or displaying thenormalized pressure measurements in a display.
 4. Apparatus formeasuring absolute pressure in a chamber, comprising: a differentialpressure sensor that is in fluid-flow communication with the chamber andin fluid-flow communication with an atmosphere outside the chamber andthat is capable of providing a differential pressure signal that isindicative of differential pressure between the chamber and theatmosphere; an absolute pressure sensor that is in fluid flow relationwith the chamber and that is capable of providing an absolute pressuresignal that is indicative of absolute pressure in the chamber; means forreceiving the differential pressure signal and the absolute pressuresignal and for determining a correlation factor based on a quantitativedifference between an absolute pressure value sensed by the absolutepressure sensor at a particular pressure level in the chamber and adifferential pressure value sensed by the differential pressure sensorat the particular pressure level in the chamber; and means fordetermining an absolute pressure value of a second pressure level in thechamber by determining a second differential pressure value from thedifferential pressure signal provided by the differential pressuresensor at the second pressure level in the chamber and adjusting thesecond differential pressure value by the correlation factor to providethe absolute pressure value of the second pressure level in the chamber.5. The apparatus of claim 4, wherein said particular pressure level inthe chamber is in a range in which the differential pressure signalprovided by the differential pressure sensor is indicative of thedifferential pressure within a desired accuracy and in which theabsolute pressure signal provided by the absolute pressure sensor isindicative of the absolute pressure within a desired accuracy.
 6. Theapparatus of claim 5, wherein said second pressure level is not withinthe range in which the absolute pressure signal provided by the absolutepressure sensor is indicative of the absolute pressure within saiddesired accuracy.
 7. Apparatus for measuring absolute pressure in achamber, comprising: means for providing a differential pressure valuethat is indicative of differential between pressure inside the chamberand pressure outside the chamber; means for providing an absolutepressure value that is indicative of pressure inside the chamber; meansfor determining a correlation factor between the differential pressurevalue and the absolute pressure value at a particular first pressure inthe chamber; and means for providing an absolute pressure value for asecond particular pressure in the chamber by adjusting a differentialpressure value, which is indicative of the differential between thepressure in the chamber at said second particular pressure and thepressure outside the chamber, with the correlation factor to provide theabsolute pressure value for the second particular pressure in thechamber.